1. Field of the Invention
The present invention relates to a process for fabricating a thin film transistor (TFT) device in a liquid crystal display (LCD) device, and more particularly, to a method of forming a complementary metal oxide semiconductor thin film transistor (CMOS TFT) device.
2. Description of the Related Art
In TFT-LCDs, a polycrystalline silicon (p-Si) TFT formed on a quartz substrate, or an amorphous silicon (a-Si) TFT formed on a glass substrate, is widely used. The TFTs in TFT-LCDs are used in one instance for a TFT matrix in a display portion and in another instance for formation of an outer circumferential circuit on a common substrate for driving said TFT matrix. In the former instance, an N-channel TFT is used, and in the latter instance, a CMOS TFT semiconductor circuit is used for achieving high speed operation.
The CMOS TFT device comprises an N-channel TFT and a P-channel TFT. Typically, the N-channel TFT has an LDD (lightly doped drain) structure in order to improve the hot electron effect, thereby decreasing leakage. A conventional method of forming the CMOS TFT device will be explained with reference to FIGS. 1A to 1E.
In FIG. 1A, a glass substrate 100 having an NMOS area 110 and a PMOS area 120 is provided. By performing a first patterning process using a first photomask, a first polysilicon island 130 and a second polysilicon island 135 are formed on the substrate 100. The first polysilicon island 130 is located in the NMOS area 110 and the second polysilicon island 135 is located in the PMOS area 120.
In FIG. 1A, a silicon oxide (SiOx) layer 140 is formed over the polysilicon islands 130 and 135 and the substrate 100. A metal layer (not shown) is then formed on the silicon oxide layer 140. By performing a second patterning process using a second photomask, the metal layer (not shown) is patterned to form a first gate 141 and a second gate 142 on part of the silicon oxide layer 140. The first gate 141 is located in the NMOS area 110 and the second gate 142 is located in the PMOS area 120.
In FIG. 1B, using the first gate 141 and the second gate 142 as a mask, an n−-ion implantation 150 is performed to form an n−-polysilicon film 151 in part of the first polysilicon island 130 and part of the second polysilicon island 135. The n−-polysilicon film 151 serves as an LDD (lightly doped drain) structure 151.
In FIG. 1C, by performing a third patterning process using a third photomask, a first photoresist pattern 160 is formed to cover the PMOS area 120 and part of the NMOS area 110. Then, an n+-ion implantation 170 is performed to form an n+-polysilicon film 171 in part of the first polysilicon island 130. The n+-polysilicon film 171 serves as a source/drain region. Thus, an NMOS TFT 175 is obtained. It should be noted that, referring to FIG. 1E, misalignment occurs easily in the NMOS area 110 due to use of multiple photomasks, specifically due to the use of a second photomask which is different from the third photomask. This causes the LDD structure 151 (or n+-polysilicon film) to be narrower on one side and wider on the other side. That is, the LDD structure 151 (or n−-polysilicon film) is not symmetrically located in the first polysilicon layer 130 beside the first gate 141, thereby increasing leakage current.
In FIG. 1D, the first photoresist pattern 160 is removed. By performing a fourth patterning process using a fourth photomask, a second photoresist pattern 180 is formed to cover the NMOS area 110. Then, a p+-ion implantation 190 is performed to form a p+-polysilicon film 191 in part of the second polysilicon island 135. The p+-polysilicon film 191 serves as a source/drain region. Thus, a PMOS TFT 195 is obtained. Lastly, the second photoresist pattern 180 is removed, as shown as FIG. 1E.
The conventional method uses two different photomasks to define the gate and the LDD structure, often resulting in misalignment. This causes an asymmetrical LDD structure in the NMOS TFT, thereby increasing leakage current. In addition, the conventional method requires four photomasks, which is complicated and expensive.